1. Field of the Invention
The present invention relates to wavelength division multiplexing (hereinafter called WDM) optical transmission, particularly relates to WDM optical transmission method and system wherein transmission characteristics are not influenced by the increase or the decrease of signal lights.
2. Description of the Related Prior Art
In a WDM optical transmission system, plural signal lights the respective wavelengths of which are different are multiplexed and are transmitted via an optical fiber. Therefore, a data amount propagated in one optical fiber rapidly increases. As shown in FIG. 1, a conventional type 32-channel WDM optical transmission system is provided with thirty-two optical transmitters 1—1 to 1-32, an optical multiplexer 2, an optical transmission line 3, an optical demultiplexer 4 and thirty-two optical receivers 5-1 to 5-32. The optical transmitter 1—1 modulates a laser beam according to an electric signal i1 and outputs a signal light A1. The other optical transmitters also respectively output a signal light of each different wavelength (for example, an interval between wavelengths is 0.4 nm). The optical multiplexer 2 multiplexes signal lights A1 to A32 and outputs a wavelength multiplexed signal light B to the optical transmission line 3. The optical transmission line 3 is provided with an optical fiber 3a and an erbium-doped fiber amplifier (hereinafter called EDFA) 3b. The optical demultiplexer 4 demultiplexes the multiplexed signal light B transmitted via the optical transmission line 3 every wavelength and outputs signal lights of thirty-two channels C1 to C32. The optical receivers 5-1 to 5-32 demodulate these signal lights to electric signals.
EDFA arranged on the optical transmission line of the above-mentioned conventional type WDM optical transmission system has a wavelength characteristic. That is, the amplification degree of EDFA slightly has dependency upon a wavelength. A wavelength multiplexed signal light propagated on the optical transmission line has difference in the level of a signal light according to a wavelength. Therefore, in the 32-channel WDM optical transmission system shown in FIG. 1, EDFA is regulated beforehand so that no difference in a level is made between thirty-two signal lights different in a wavelength (that is, the wavelength characteristic is flat) when thirty-two signal lights are transmitted.
However, when the operation of this WDM optical transmission system is started, a case that all signal lights of 32 channels are transmitted is rare. Ordinarily, a signal light often increases by degrees as subscribers and the demand increase. Therefore, it means the increase of initial investment that channels unused when the operation of the above-mentioned system is started are installed. When the power of light input to the optical transmission line is smaller than that of 32-channel signal lights, the wavelength characteristic of EDFA may be not flat and the level of a signal light the wavelength of which is short of a wavelength multiplexed signal light may be deteriorated. When an input level to the optical receiver is insufficient, the level of an electric signal may be deteriorated and no electric signal may be output.
Japanese published unexamined patent application No. Hei 11-252047 discloses a 16-channel optical wavelength multiplexing system wherein when an unused channel exists, light of a wavelength different from that of a signal light is transmitted together with the signal light to prevent the characteristic of the signal light from being deteriorated and to control the increase of initial investment. However, in this system, as light of one wavelength different from that of a signal light is transmitted, the gain profile of EDFA changes when the number of channels is increased and it may influence the characteristic of another signal light.